Try
add_filter( 'acf/validate_field_group', 'toind_alter_flexible_content', 20 );
I’m not sure this will work at all. After digging through the code of ACF I do know that neither acf/load_field_group or acf/get_field_group are used when loading a field group from a .json file. The above hook was the only place that a change might be possible.
I don’t know what other issues there might be. For example json files may be loaded before $GLOBALS[‘post_type’] is set, or you could still be seeing interference from the WP user metabox order, or for that matter, the filter hook will not work at all. I tested and it did work for me.
